{"id":52213,"date":"2026-11-01T10:50:39","date_gmt":"2026-11-01T10:50:39","guid":{"rendered":"http:\/\/siratek.net\/index.php\/tutorials\/data-logger\/"},"modified":"2026-11-01T10:50:39","modified_gmt":"2026-11-01T10:50:39","slug":"data-logger","status":"publish","type":"siratek_tutorial","link":"https:\/\/siratek.net\/index.php\/tutorials\/data-logger\/","title":{"rendered":"Project, a Standalone Data Logger"},"content":{"rendered":"

<\/p>\n

What you will build<\/h2>\n

A box you leave somewhere for a week. It records a temperature every minute with a real date and time against it, and when you come back the card opens in a spreadsheet and draws a graph.<\/p>\n

No laptop, no network, no watching. That is the whole point: this is the project that lets you find out what actually happened to the greenhouse overnight, or how cold the store room gets before dawn.<\/p>\n

It is also the project where the failures matter<\/strong>. A logger that stops on Tuesday and tells nobody has wasted a week, so a good part of this page is about noticing and surviving.<\/p>\n

Before you begin<\/h2>\n

Parts<\/h3>\n\n\n\n\n\n\n\n\n\n\n\n
Part<\/th>\nSiratek product<\/th>\nIts own tutorial<\/th>\n<\/tr>\n<\/thead>\n
Arduino Uno<\/td>\n2219<\/td>\nGetting Started with Arduino Uno<\/em><\/td>\n<\/tr>\n
Micro SD card module<\/td>\n6573<\/td>\nLogging Data to an SD Card<\/em><\/td>\n<\/tr>\n
DS3231 real time clock<\/td>\n2522<\/td>\nKeeping Time with a DS3231<\/em><\/td>\n<\/tr>\n
DS18B20 waterproof probe<\/td>\n49704<\/td>\nWaterproof Temperature with DS18B20<\/em><\/td>\n<\/tr>\n
4.7 kilohm resistor<\/td>\n<\/td>\nRequired by the probe<\/td>\n<\/tr>\n
Micro SD card, 32 GB or smaller, FAT32<\/td>\n<\/td>\n<\/td>\n<\/tr>\n
Breadboard and jumper wires<\/td>\n<\/td>\nUsing a Breadboard<\/em><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

What shares what<\/h3>\n

Three devices, three different ways of talking:<\/p>\n\n\n\n\n\n\n\n
Device<\/th>\nBus<\/th>\nPins<\/th>\n<\/tr>\n<\/thead>\n
SD card<\/td>\nSPI<\/td>\nD10<\/code> to D13<\/code><\/td>\n<\/tr>\n
DS3231<\/td>\nI2C<\/td>\nA4<\/code>, A5<\/code><\/td>\n<\/tr>\n
DS18B20<\/td>\n1-Wire<\/td>\nD2<\/code><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

They do not conflict, which is pleasant and worth noticing: SPI, I2C and 1-Wire are three separate arrangements and can all run at once. Between them they take six pins, leaving most of the board free.<\/p>\n

The DS3231 is at address 0x68. Nothing else here is on I2C, so there is no clash \u2014 but if you add an MPU-6050 later there will be, and Keeping Time with a DS3231<\/em> explains the fix.<\/p>\n

The power budget<\/h3>\n\n\n\n\n\n\n\n\n
Part<\/th>\nTypical<\/th>\nPeak<\/th>\n<\/tr>\n<\/thead>\n
Uno<\/td>\n50 mA<\/td>\n<\/td>\n<\/tr>\n
SD card<\/td>\na few mA idle<\/td>\nup to ~100 mA while writing<\/strong><\/td>\n<\/tr>\n
DS3231<\/td>\nunder 1 mA<\/td>\n<\/td>\n<\/tr>\n
DS18B20<\/td>\nabout 1 mA<\/td>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

On USB that is comfortable. The peak matters because a card write is exactly when a brownout is most damaging: a reset mid-write can leave the file system, not just the line, in a bad state.<\/p>\n

The sketch below opens, writes and closes the file every single line for that reason. It is slower, and at one reading a minute the speed is irrelevant.<\/p>\n

Choosing the interval<\/h3>\n

Worth a moment\u2019s arithmetic before you start.<\/p>\n\n\n\n\n\n\n\n
Interval<\/th>\nLines per day<\/th>\nA week<\/th>\n<\/tr>\n<\/thead>\n
1 second<\/td>\n86,400<\/td>\n600,000<\/td>\n<\/tr>\n
1 minute<\/td>\n1,440<\/td>\n10,080<\/td>\n<\/tr>\n
10 minutes<\/td>\n144<\/td>\n1,008<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

A week at one minute is ten thousand lines, which is about 300 KB and opens instantly in a spreadsheet. A week at one second is six hundred thousand lines, which most spreadsheets will refuse or struggle with, and tells you nothing a minute\u2019s average would not.<\/p>\n

One minute is the right default<\/strong> for temperature. Faster only if the thing you are measuring genuinely changes faster.<\/p>\n

How it works<\/h2>\n

Four things happen, at three different rates:<\/p>\n\n\n\n\n\n\n\n\n
Job<\/th>\nHow often<\/th>\n<\/tr>\n<\/thead>\n
Read the probe and the clock<\/td>\nEvery minute<\/td>\n<\/tr>\n
Write a line to the card<\/td>\nEvery minute<\/td>\n<\/tr>\n
Write a heartbeat line<\/td>\nEvery hour, whether or not anything changed<\/td>\n<\/tr>\n
Blink the LED<\/td>\nEvery reading<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The heartbeat<\/strong> is the part people leave out and then regret. A log that only records events cannot distinguish \u201cnothing happened\u201d from \u201cthe logger stopped\u201d. An hourly line, even an uninteresting one, means a gap in the file tells you exactly when something went wrong.<\/p>\n

Pin reference<\/h2>\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n
Device<\/th>\nPin<\/th>\nConnect to<\/th>\n<\/tr>\n<\/thead>\n
SD module<\/td>\nGND<\/code><\/td>\nUno GND<\/code><\/td>\n<\/tr>\n
SD module<\/td>\nVCC<\/code><\/td>\nUno 5V<\/code><\/td>\n<\/tr>\n
SD module<\/td>\nMISO<\/code><\/td>\nUno D12<\/code><\/td>\n<\/tr>\n
SD module<\/td>\nMOSI<\/code><\/td>\nUno D11<\/code><\/td>\n<\/tr>\n
SD module<\/td>\nSCK<\/code><\/td>\nUno D13<\/code><\/td>\n<\/tr>\n
SD module<\/td>\nCS<\/code><\/td>\nUno D10<\/code><\/td>\n<\/tr>\n
DS3231<\/td>\nGND<\/code>, VCC<\/code><\/td>\nUno GND<\/code>, 5V<\/code><\/td>\n<\/tr>\n
DS3231<\/td>\nSDA<\/code>, SCL<\/code><\/td>\nUno A4<\/code>, A5<\/code><\/td>\n<\/tr>\n
DS18B20 red<\/td>\nUno 5V<\/code><\/td>\n<\/td>\n<\/tr>\n
DS18B20 black<\/td>\nUno GND<\/code><\/td>\n<\/td>\n<\/tr>\n
DS18B20 yellow<\/td>\nUno D2<\/code>, and 4.7 k to 5V<\/code><\/strong><\/td>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
\"A
Three devices, three different buses, six pins. The 4.7 k pull-up is not supplied with the probe.<\/figcaption><\/figure>\n

Build it in stages<\/h2>\n

Stage 1: each part alone<\/h3>\n

Run the minimal example from each of the three component tutorials, one at a time. Specifically, confirm:<\/p>\n

    \n
  • The card initialises and lists its contents.<\/li>\n
  • The clock returns a sensible time, with lostPower()<\/code> handled so it is not reset on every boot.<\/li>\n
  • The probe reports a device count of 1, not 0, and a plausible temperature.<\/li>\n<\/ul>\n

    A device count of 0 means the 4.7 k resistor is missing. A -127.00<\/code> means the same thing.<\/p>\n

    Stage 2: all three, to serial<\/h3>\n

    Combine them with no card writing at all: read the clock and the probe once a minute and print a CSV line to the serial monitor.<\/p>\n

    Watch it for five minutes. Then unplug the probe and watch what happens, because that is a rehearsal for the thing that will go wrong in the field.<\/p>\n

    Stage 3: write to the card<\/h3>\n

    Add the file. The complete sketch below.<\/p>\n

    The complete sketch<\/h2>\n
    \/\/ Siratek: a standalone temperature logger.\n\/\/ Writes one CSV line a minute, with a real timestamp, and keeps going\n\/\/ when a part fails.\n\n#include <SPI.h>\n#include <SD.h>\n#include <Wire.h>\n#include <RTClib.h>\n#include <OneWire.h>\n#include <DallasTemperature.h>\n\nconst int chipSelect = 10;\nconst int ledPin = LED_BUILTIN;\nconst char* FILENAME = \"LOG.CSV\";          \/\/ 8.3 names only\n\nRTC_DS3231 rtc;\nOneWire oneWire(2);\nDallasTemperature probe(&oneWire);\n\nconst unsigned long INTERVAL  = 60000UL;   \/\/ one minute\nconst unsigned long HEARTBEAT = 3600000UL; \/\/ one hour\n\nunsigned long lastReading = 0;\nunsigned long lastHeartbeat = 0;\n\nbool cardReady = false;\nbool clockReady = false;\n\nunsigned long linesWritten = 0;\nunsigned long writeFailures = 0;\n\nvoid setup() {\n  pinMode(ledPin, OUTPUT);\n  Serial.begin(9600);\n  Serial.println(F(\"Siratek: data logger starting.\"));\n\n  clockReady = rtc.begin();\n  if (!clockReady) {\n    Serial.println(F(\"  no clock: timestamps will be elapsed minutes\"));\n  } else if (rtc.lostPower()) {\n    Serial.println(F(\"  clock had lost power, setting from compile time\"));\n    rtc.adjust(DateTime(F(__DATE__), F(__TIME__)));\n  }\n\n  probe.begin();\n  Serial.print(F(\"  probes found: \"));\n  Serial.println(probe.getDeviceCount());\n  if (probe.getDeviceCount() == 0) {\n    Serial.println(F(\"  check the 4.7k between data and 5V\"));\n  }\n\n  cardReady = SD.begin(chipSelect);\n  if (!cardReady) {\n    Serial.println(F(\"  no card: logging disabled, serial only\"));\n  } else if (!SD.exists(FILENAME)) {\n    File f = SD.open(FILENAME, FILE_WRITE);\n    if (f) {\n      f.println(F(\"timestamp,temperature_c,note\"));\n      f.close();\n      Serial.println(F(\"  created LOG.CSV with a header\"));\n    }\n  }\n\n  \/\/ Take the first reading straight away rather than waiting a minute.\n  lastReading = millis() - INTERVAL;\n  lastHeartbeat = millis();\n}\n\n\/\/ Build a timestamp, or an elapsed count if the clock is missing.\nvoid appendTimestamp(String& line) {\n  if (!clockReady) {\n    line += F(\"elapsed_\");\n    line += String(millis() \/ 60000UL);\n    return;\n  }\n\n  DateTime now = rtc.now();\n  char buf[20];\n  snprintf(buf, sizeof(buf), \"%04d-%02d-%02d %02d:%02d:%02d\",\n           now.year(), now.month(), now.day(),\n           now.hour(), now.minute(), now.second());\n  line += buf;\n}\n\nvoid writeLine(const String& line) {\n  Serial.println(line);\n\n  if (!cardReady) {\n    return;\n  }\n\n  \/\/ Open, write, close, every time: a closed file survives a power cut.\n  File f = SD.open(FILENAME, FILE_WRITE);\n  if (f) {\n    f.println(line);\n    f.close();\n    linesWritten++;\n  } else {\n    writeFailures++;\n    Serial.println(F(\"  could not open the file\"));\n\n    \/\/ The card may have been removed. Try to pick it up again next time.\n    if (writeFailures > 3) {\n      cardReady = SD.begin(chipSelect);\n      writeFailures = 0;\n    }\n  }\n}\n\nvoid takeReading() {\n  digitalWrite(ledPin, HIGH);\n\n  probe.requestTemperatures();\n  float c = probe.getTempCByIndex(0);\n\n  String line = \"\";\n  appendTimestamp(line);\n  line += \",\";\n\n  if (c == DEVICE_DISCONNECTED_C) {\n    line += \",probe not responding\";\n  } else {\n    line += String(c, 1);\n    line += \",\";\n  }\n\n  writeLine(line);\n  digitalWrite(ledPin, LOW);\n}\n\nvoid heartbeat() {\n  String line = \"\";\n  appendTimestamp(line);\n  line += \",,alive: \";\n  line += String(linesWritten);\n  line += \" lines\";\n  writeLine(line);\n}\n\nvoid loop() {\n  if (millis() - lastReading >= INTERVAL) {\n    lastReading = millis();\n    takeReading();\n  }\n\n  if (millis() - lastHeartbeat >= HEARTBEAT) {\n    lastHeartbeat = millis();\n    heartbeat();\n  }\n}<\/code><\/pre>\n
    Four decisions that make this a logger rather than a demonstration<\/h3>\n
    It runs without any of its parts.<\/strong> No card, it logs to serial. No clock, it timestamps with elapsed minutes. No probe, it writes a line saying so. A logger that refuses to start because one thing is missing has thrown away everything else it could have recorded.<\/p>\n
    The probe failure is recorded, not skipped.<\/strong> A missing line is ambiguous; a line saying \u201cprobe not responding\u201d is evidence. When you open the file in a week, the difference matters.<\/p>\n
    It tries to recover the card.<\/strong> Three failures in a row and it calls SD.begin()<\/code> again, which picks up a card that was removed and put back, or one that glitched. It is four lines and it turns a permanent failure into a gap.<\/p>\n
    The heartbeat proves it is alive.<\/strong> An hourly line means a gap in the file has a start time and an end time. Without it you know only that the file stops.<\/p>\n
    A note on String<\/code><\/h3>\n
    This sketch uses String<\/code> to build each line, and at one line a minute that is fine: the heap has time to settle between allocations.<\/p>\n
    If you log every second, replace it with snprintf()<\/code> into a fixed char<\/code> buffer. Logging Data to an SD Card<\/em> explains why, and the symptom of ignoring it is a sketch that runs for hours and then freezes.<\/p>\n
    Testing it<\/h2>\n
    Before leaving it anywhere, do these five. Each one rehearses a real failure.<\/p>\n\n\n\n\n\n\n\n\n\n
    Try this<\/th>\nIt should<\/th>\n<\/tr>\n<\/thead>\n
    Run it for five minutes<\/td>\nFive lines, a minute apart, with sensible times<\/td>\n<\/tr>\n
    Pull the probe out<\/td>\nKeep logging, with \u201cprobe not responding\u201d<\/td>\n<\/tr>\n
    Put the probe back<\/td>\nResume recording temperatures<\/td>\n<\/tr>\n
    Pull the card out and put it back<\/td>\nReport failures, then recover within a few minutes<\/td>\n<\/tr>\n
    Unplug the power mid-run<\/td>\nOn restart, continue appending to the same file<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
    The last is the important one. Unplug it, plug it back in, and open the card: the header should appear once, not twice, and the earlier lines should still be there.<\/p>\n
    Then take the card to a computer, open LOG.CSV<\/code> in a spreadsheet, and chart the temperature column. That is the project working.<\/p>\n
    Making it yours<\/h2>\n
      \n
    • More sensors.<\/strong> A second DS18B20 on the same pin and the same resistor, which is the whole point of 1-Wire. Add a column per probe.<\/li>\n
    • Humidity.<\/strong> A DHT22 on a spare pin. The weather station project has the reading code.<\/li>\n
    • Run it on batteries.<\/strong> This is the real upgrade, and it leads straight to Running an ATmega328P Without the Board<\/em>: the Uno\u2019s regulator and USB chip draw far more than the logging does. A bare chip asleep between readings runs for months.<\/li>\n
    • A display.<\/strong> An OLED showing the current reading and the line count, so you can tell at a glance that it is working without pulling the card.<\/li>\n
    • A watchdog.<\/strong> Making a Project Survive Unattended<\/em> covers it, and this is exactly the kind of project it exists for.<\/li>\n<\/ul>\n
      Troubleshooting<\/h2>\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n
      Symptom<\/th>\nLikely cause<\/th>\nCheck or fix<\/th>\n<\/tr>\n<\/thead>\n
      Card will not initialise<\/td>\nToo large, or not FAT32<\/td>\n32 GB or smaller, FAT32<\/td>\n<\/tr>\n
      Card fails intermittently<\/td>\nLong jumper leads, or a cheap card<\/td>\nShorten them; try another card<\/td>\n<\/tr>\n
      File never appears<\/td>\nFilename longer than 8.3<\/td>\nLOG.CSV<\/code>, not templog.csv<\/code><\/td>\n<\/tr>\n
      Header appears twice<\/td>\nWritten unconditionally<\/td>\nGuard it with SD.exists()<\/code><\/td>\n<\/tr>\n
      Temperature reads -127<\/td>\nNo 4.7 k pull-up<\/td>\nBetween the data wire and 5V<\/code><\/td>\n<\/tr>\n
      Probe count is 0<\/td>\nSame cause<\/td>\nFit the resistor<\/td>\n<\/tr>\n
      Timestamps all identical<\/td>\nClock not set<\/td>\nCheck lostPower()<\/code> handling<\/td>\n<\/tr>\n
      Timestamps reset on every boot<\/td>\nadjust()<\/code> called unconditionally<\/td>\nGuard it with lostPower()<\/code><\/td>\n<\/tr>\n
      Timestamps say elapsed_<\/code><\/td>\nClock not found<\/td>\nScan the bus; expect 0x68<\/td>\n<\/tr>\n
      File corrupted after a power cut<\/td>\nFile left open<\/td>\nOpen, write, close each line<\/td>\n<\/tr>\n
      Logger stopped and you cannot tell when<\/td>\nNo heartbeat<\/td>\nAn hourly line gives you the gap<\/td>\n<\/tr>\n
      Runs for hours then freezes<\/td>\nString<\/code> fragmenting the heap<\/td>\nsnprintf()<\/code> into a fixed buffer<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
      Sources and attribution<\/h2>\n
      All text, wiring guidance and code in this project are original Siratek work.<\/p>\n
        \n
      • Component behaviour is as described in Logging Data to an SD Card<\/em>, Keeping Time with a DS3231<\/em> and Waterproof Temperature with DS18B20<\/em>, including the 8.3 filename limit, the lostPower()<\/code> behaviour and the probe\u2019s required pull-up resistor.<\/li>\n<\/ul>\n
        Current figures are typical and vary with the card. No third-party tutorial text, diagrams, photographs or code has been reproduced here.<\/p>\n
        <\/p>\n<\/body>","protected":false},"excerpt":{"rendered":"
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